A general method to separate oxygen in air is the cryogenic distillation method which rapidly decreases the temperature of air at room temperature to liquefy air, and separates oxygen therefrom by distillation.
However, the cryogenic distillation method is disadvantageous in that an excessively large amount of cost is required for purchasing and installing apparatuses since a large sized-coding apparatus for rapid coding air and a large sized-distillation bed for distilling the coded air should be provided.
Comparing the cryogenic distillation method with the adsorption methods, the cryogenic distillation method has a drawback in that much more costs are required, but nevertheless is preferred to the adsorption method since it also has an advantage that oxygen having a high purity of 99% or more can be produced. Due to the massive equipment and huge investment cost, however, it is almost impossible for small and medium scales to employ such equipments.
Thus, a main method currently used is the adsorption method to separate elements by using a solid on which micro pores corresponding to a bulk of specific elements in air are formed. Zeolite is mainly used as adsorbent in the adsorption method because zeolite acts as strong adsorbent against nitrogen and as weak adsorbent against oxygen.
Therefore, once air is supplied to zeolite, nitrogen is adsorbed into the zeolite and oxygen is transmitted and discharged, thereby producing oxygen with nitrogen remarkably excluded. At this time, thus produced oxygen includes a very little amount of nitrogen which are not removed from the zeolite and argon which is not adsorbed due to its adsorptability similar to oxygen.
Accordingly, the oxygen production system utilizing the conventional adsorption method is consisted of a zeolite bed having an inner space filled with zeolite. In a case, two to four of zeolite beds are used in order to raise the purity of oxygen.
However, oxygen produced by the above adsorption-type oxygen production system has the maximum purity of 95% only due to difficulty of separating argon. Thus, oxygen manufacturers have developed a process for producing oxygen having a high purity of 99% or more with using the above system since 1980s.
And, at present, BOC and Crew Technology Division Amstrong Rabortory Co., Ltd. of the United State of America and Suminoto Seike Co., Ltd of Japan distinguished themselves as the leading companies in the industry field, and they are planning to commercialize a system which can produce oxygen having the high impurity of 99.7%.
However, the oxygen production systems provided in these makers comprises an adsorption process for separating bulk and an adsorption process for purification, and once the adsorption process for separating bulk is completely finished, the adsorption process for purification should be performed. Thus, the biggest problem in these systems is that the unit production cost for oxygen is high due to operation of each adsorption process.
On the other hand, nitrogen having high purity can be produced by the adsorption method. In this case, a carbon molecular sieve (CMS) is used as adsorbent because oxygen could be adsorbed at tens to hundreds times higher rate than nitrogen or argon.
That is, due to differences in the adsorption rates to C(4S among oxygen, nitrogen, and argon, oxygen is adsorbed rapidly to CMS, while nitrogen and argon are not adsorbed, and are penetrated through CMS, thereby to produce nitrogen of high purity.
This nitrogen production system employing CMS is generally consisted of a CMS bed filled with (MS. In a case, two to four of CMS beds are used in order to raise the purity and output of nitrogen.
On the other hand, recently, a new process employing the zeolite bed system as described above has been developed to produce oxygen having a purity of 99% or more by first producing oxygen having a high purity of 90˜95% with including impurities such as nitrogen and argon, and then filtrating the oxygen under (MS bed system as described above.
However, this process employs a multi-step system in which the zeolite bed system and the CMS bed system are separately provided and independently operated. Accordingly, this system should be equipped with all the individual systems, and also doubled costs for operation and energy should be paid for individual independent operation. Further, the raw material should be passed through the individual systems, and so the recovery rate of oxygen is sharply lowered.